This invention relates to a process measurement instrument and, more particularly, to a split intrinsic safety barrier.
Knowledge of level and flow and the like in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making such measurements. Many of these devices use an active sensing element which is exposed to the process.
Two wire measurement instruments are frequently installed in potentially explosive atmospheres. Two common safety considerations are to design the entity as intrinsically safe or use an explosion proof device. As an intrinsically safe entity, typically a diode barrier in the control room limits the energy available to the instrument. As an explosion proof entity an instrument housing prevents any explosive faults from propagating into the surrounding explosive atmosphere. However, the explosion proof housing does not protect the external active element from being a point for generating explosive faults. For explosive proof applications, the active element needs a secondary protective device.
One solution is to use a fuse protected diode safety barrier inside the explosion proof housing to limit the energy at the active element. The fuse protected diode safety barrier may include shunt diodes or diode chains, including zener diodes, protected by fuses and resistors. The diodes limit voltage, while the resistors limit current. The fuses limit excessive surge currents. One known option is to place the fuse protected diode safety barrier in the housing between the two-wire connection and the instrument electronic circuitry. In this configuration, the two-wire DC current requirement and barrier grounding requirements introduce unwanted ground loops. Another known solution is to place the fuse protected diode safety barrier between the electronics and the probe with AC coupling to eliminate ground loops. However, the fuse inductance can cause unwanted distortions in the measured signal.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention there is provided a process instrument with a split intrinsic safety barrier.
In one aspect of the invention, a process instrument comprises a housing, a terminal in the housing for connection to a two-wire process loop and an active element for sensing a characteristic of a process. A control circuit is disposed between the terminal and the active element for measuring the sensed characteristic and controlling the two-wire process loop responsive to the sensed characteristic. A split intrinsic safety barrier comprises current limiters electrically connected between the terminal and the control circuit and a diode safety barrier electrically connected between the control circuit and the active element.
In accordance with the invention, the active element may comprise a guided wave radar transmission line, a capacitance probe, a through air radar transducer or an ultrasonic transducer.
It is a feature of the invention that the current limiters comprise fuses.
It is another feature of the invention that the diode safety barrier comprises a plurality of diodes connected across the active element. The diodes may comprise a plurality of pairs of reverse connected diodes or a plurality of zener diodes. The diode safety barrier is AC coupled to the control circuit. It is another feature of the invention that the diode safety barrier comprises a current limit resistor.
Further features of the invention will be readily apparent from the specification and from the drawings.